A variety of automotive exhaust gas sensors are known. FIG. 1 illustrates a prior art sealed, watertight oxygen gas sensor 10 including electrical leads 12, 14, 16 connecting to a heating element 18, and to internal and external electrodes 20, 22 surrounding a conical-shaped electrolyte body 24 made of zirconia. The exhaust sensor housing includes a lower tubular louvered shell 26, a thicker middle shell 28 and an upper tubular shell 30. The conical-shaped electrolyte body and heater are held in position by a sub-assembly including an upper ceramic insulator through which a heating element passes. An annular finger 32 of the middle shell is crimped over a first shoulder 34 formed in the upper ceramic insulator to hold the electrolyte body and heater in position. A belleville washer 36 is positioned above the middle housing and located to be biased against a shoulder 38 formed in the upper tubular shell and a second shoulder 35 formed on the upper ceramic insulator body. During the operation of the oxygen sensor, the assembly undergoes thermal expansion and contraction. The belleville washer is positioned to provide a downward force on the upper ceramic insulator body and thus the heating element 18 and electrolyte body 24 to maintain their position and electrical contact within the sensor. The downward force of the belleville washer also maintains a tight seal by way of a lower annular gasket 37 positioned between a lower sloped shoulder 39 of the electrolyte body and a sloped shoulder 41 of the middle shell to provide a gastight seal.
However, this design illustrated in FIG. 1 is not optimal. When the annular finger 32 of the middle shell is crimped over the first shoulder 34 of the upper ceramic insulator, the crimped finger can bind the ceramic insulator so that the insulator moves with the expansion of the middle shell. However, since the ceramic insulator is stuck in the middle shell, the belleville washer 36 cannot transmit a sufficient force to maintain a seal on the lower gasket 37 between the lower sloped shoulder 39 of the electrolyte body and the lower sloped shoulder 41 of the middle shell. Thus, this design can result in exhaust gas entering the housing and contaminating the air reference gas for the sensing device. The exhaust gas entering through the lower seal results in displacement of reference oxygen and can result in a lower oxygen concentration on the wrong side (the side of the inner electrolyte body wall forming the conical-shaped cavity) of the electrolyte body thus resulting in a negative signal shift being produced by the oxygen sensor.
FIG. 2 illustrates a water resistant design in which the upper shell 50 is not hermetically welded to the middle shell 52, but is mechanically secured with a locking washer 54 located above a crimp finger 56 of the middle shell. The upper shell 50 is pushed downward over the locking washer. The locking washer 54 provides a friction fit and bends downward as the upper shell 50 is pushed down, but digs into the upper shell 50 when it is lifted upward. This type of sensor has a air reference that is open to the atmosphere. This design is much less sensitive to exhaust gas leakage into the air reference side of the sensing element because of the dilution of leaking exhaust gas by the atmosphere. This design utilizes a wave washer disk spring (shown in FIG. 3) 58 in the internal assembly. The assembly includes an upper ceramic insulator 60 having a upper sloped shoulder 59 and a flat lower shoulder 61. A hole is formed in the upper ceramic insulator for receiving a heating element 18. A terminal post 62 for the heating element extends through the electrolyte body and includes an outwardly extending annular flat 72. A gripper 63 is carried inside the terminal post. The conical-shaped electrolyte body 64 is surrounded by inner and outer electrodes 66, 68. The conical-shaped electrolyte body includes a lower sloped shoulder 76 which engages an annular lower gasket 70 resting on a lower sloped shoulder 78 of the middle shell. An open nipple of an upper gasket 74 extends into the open end of the conical-shaped electrolyte body and also includes an annular flat which rests against an upper shoulder of the electrolyte body. A wave washer spring 58 is positioned between an upper ceramic insulator 60 and the terminal post 62. An annular finger 56 of the middle shell is crimped over the upper ceramic insulator 60. The vented air reference chamber provides the sensor with a continuous source of reference oxygen and therefore this type of sensor is less sensitive to the problems associated with "watertight" oxygen sensors having internal exhaust leaks. Further, application of wave-type washers have shown limited resiliency and are only capable of exerting a load of about 200-300 lbs.
Thus, heretofore, there has been a need for a watertight, sealed automotive exhaust gas sensor which overcomes the deficiencies of the prior art.